1. Field of the Invention
The present invention relates to a current excitation light-emitting element, a light-emitting device, an electronic appliance each having the light-emitting element, and method of manufacturing the same.
2. Description of the Related Art
In recent years, a light-emitting element utilizing electroluminescence has been actively researched and developed. In a basic structure of such a light-emitting element, a substance with a light-emitting property is sandwiched between a pair of electrodes. When voltage is applied to this element, light emission can be obtained from the substance with a light-emitting property.
Since such a light-emitting element is a self-luminous type, there are advantages in that it has higher visibility of pixels than liquid crystal displays, there is no need to use a backlight, and the like. Thus, such a light-emitting element is considered to be suitable as a flat panel display element. In addition, such a light-emitting element can be manufactured to be thin and light-weight, which is also a great advantage. Moreover, very high response speed is also one of features of such a light-emitting element.
Furthermore, since such a light-emitting element can be formed in a film form, planar light emission can be easily obtained when a large-area element is formed. This cannot be easily achieved with a point light source typified by an incandescent lamp or an LED, or a linear light source typified by a fluorescent lamp, and thus, such a light-emitting element has high utility value as a plane light source which is applicable to a lighting system or the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a substance with a light-emitting property.
When an organic compound is used for a substance with a light-emitting property, electrons and holes are injected into a layer containing an organic compound with a light-emitting property from a pair of electrodes by voltage application to a light-emitting element, so that current flows therethrough. Then, when carriers (electrons and holes) are recombined, the organic compound with a light-emitting property is in an excited state, and when the excited state returns to a ground state, light is emitted. Because of such a mechanism, such a light-emitting element is referred to as a current excitation light-emitting element.
Note that there are a singlet excited state and a triplet excited state as the types of the excited states obtained by an organic compound. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a lot of problems depending on a material, and in order to solve the problems, improvement of an element structure, development of a material, and the like have been carried out.
For example, in Non-Patent Document 1 (Tetsuo Tsutsui and eight others, Japanese Journal of Applied Physics, vol. 38, L1502-1504 (1999)), a hole blocking layer is provided, so that a light-emitting element using a phosphorescent material efficiently emits light.